364 BOTANICAL GAZETTE [NOVEMBER 
of the frostless season. For the intensity factor of temperature 
effectiveness it is necessary to employ as data, not the temperature 
of any single instant, nor the mean temperature of any single day, 
but what has been termed the normal mean temperature for each 
day of the frostless season. Fortunately, these data have been 
calculated by BicrLow and have been made available through 
publication by the U.S. Weather Bureau.%* These have been 
here employed. 
The plan of this study is: (1) to sum the normal daily mean 
temperatures of each station considered, for the period of the 
average frostless season; (2) to sum the temperature efficiencies 
corresponding, respectively, to the normal daily means and to the 
adopted coefficient for 10° of variation; (3) to plot both sets of 
temperature indices so obtained in the form of charts; and (4) to 
compare the form and location of the climatic areas or zones thus 
exhibited. 
The temperature indices 
As in the case of direct temperature summations, so in that 
of the summation of temperature efficiencies, it is necessary to 
establish a temperature which may be taken as a starting-point. 
This should approximate the temperature at which general plant 
growth is evident and should be chosen according to the same 
criteria as are employed by phenology in similar cases. For both 
series of summations we have taken the rate of growth and develop- 
ment as unity with a daily mean temperature of 40° F. (4.4° C.). 
Normal daily means below 40° F. do not occur in BIGELOW’S 
tables. If they did occur, growth on such dates would be taken as 
nil, on the basis of our assumptions. : 
For each of the direct summations, the normal daily mean bid 
39, for the date next following the average date of the last frost in 
spring, is taken as the first term. To this are added the normal 
daily means, each decreased by 39,.for all dates up to and including 
the average date of the last frost in autumn. Practically, the 
summation of the unmodified normal daily means was first made 
* BIGELow, F, H., The daily normal temperature and daily normal precipitation 
of the United States. U.S. Dept. Agric., Weather Bureau, Bull. R. 1908. 
